1. Field of the Invention
This invention relates to an amorphous memory device for an electrically alterable read-only memory and, more particularly, to such a device which has a stable threshold voltage over a long lifetime and for a large temperature range.
2. Description of the Prior Art
Memory cells for electrically alterable read-only memories and other non-volatile memories can be formed from amorphous semiconductor devices which are capable of being switched to and from a low-resistance crystalline state. A particular type of memory switching amorphous semiconductor material is the tellurium based chalcogenide class materials which have the general formula Ge.sub.A Te.sub.B X.sub.C Y.sub.D. Such amorphous high-resistance semiconductor materials can be placed between a pair of spaced apart electrodes. By applying a voltage pulse (between the electrodes) of greater amplitude than the threshold voltage of the amorphous material, a low-resistance filament is formed between the electrodes. If this pulse is current limited and of proper duration, a conducting crystalline filament will form and then remain between the electrodes after the pulse is removed (set operation). A reset group of pulses of appropriate current and duration causes the crystalline path to return to a relatively amorphous state (reset operation).
Particular materials that may be employed are disclosed in the Ovshinsky U.S. Pat. No. 3,271,591, the Neale U.S. Pat. No. 3,600,543 and the Buckley U.S. Pat. No. 3,886,557.
Prior art designs of amorphous or ovonic memory switches have had a characteristic threshold voltage V.sub.T which is high at the first operation and in early operating life and lower thereafter ("first-fire effect") or which declines continuously throughout the life of the switch. Particularly, this decline is in response to repeated "reset" operations where the memory element is restored from its conducting condition to its high resistance condition. However, there are instances where it appears that the device lasted through 10.sup.6 set-reset cycles where the threshold voltage was observed to have a minimum low value (between 5 and 10 volts) and was relatively invariant to additional write cycles.
It appears that electromigration and thermal diffusion of the constituents of the memory material toward the different electrodes causes the steady decline in the threshold voltage. In the above-described materials, germanium and germanium-tellurium crystals appear to migrate to the negative electrode. Similarly, tellurium migrates to the positive electrode. This migration of material produces regions that are inactive in the switching process because their ratios of constituents are no longer appropriate. The region where the ratio of constituents is appropriate for switching is thus reduced in effective thickness and the threshold voltage becomes low, similarly to that of a much thinner layer.
The migration of material also produces concentration gradients. Diffusion then operates as a countervailing process, producing an equilibrium. Thermal gradients may also contribute to the process.
Memory structures with different types of layered structures have been proposed to approximate the regions of diffusion and migration that occur over many set-reset cycles. The above-referenced Buckley patent discloses an ovonic memory structure in which the threshold voltage decline is altered by placement of a tellurium layer between the positive electrode and the amorphous memory material layer. This alters the threshold voltage decline but does not eliminate it. A more complete solution to the problem of threshold voltage decline is disclosed in the Bluhm application Ser. No. 801,773, filed May 31, 1977, now U.S. Pat. No. 4,115,872, and assigned to the assignee of the present application. However, the structure disclosed therein has a voltage threshold which is temperature sensitive throughout the lifetime of the device. Temperature sensitive is defined to mean that the amorphous switching material crystallizes at temperatures as low as 70.degree. C., but can remain in an amorphous or high resistance state at temperatures up to 130.degree. C.
It is then an object of the present invention to provide an improved amorphous semiconductor device for electrically alterable read-only memories.
It is another object of the present invention to provide an amorphous memory device which has a relatively constant threshold voltage through its lifetime.
It is still a further object of the present invention to provide such an amorphous memory device which has a threshold voltage throughout its lifetime that is relatively temperature insensitive.